This invention relates to electrical feedthroughs and, more particularly, to a hermetic ceramic electrical feedthrough.
Many types of apparatus utilize an electrical feedthrough across a wall that otherwise separates two environments. The electrical feedthrough permits electrical signals and power to be conducted across the wall, but prevents any movement of mass, such as gas leakage, across the wall. As an example, an infrared sensor is typically contained in a vacuum enclosure. The sensor is cooled to cryogenic temperatures, typically about 77K or less. Output signals are conducted from the sensor to electronic devices located exterior to the vacuum enclosure, without losing the hermetic vacuum seal, via an electrical feedthrough in the wall of the enclosure.
The feedthrough is usually constructed with a plurality of electrical pins supported in an electrically insulating material such as a ceramic or a glass. The insulating material is joined to and contacts the remainder of the wall of the package structure, here the vacuum enclosure. The insulating material isolates the electrical pins from the wall and from each other.
In one common type of feedthrough, a glass is melted into the space between the electrical pin and a bore through a metallic feedthrough plate. The glass acts as the insulator. Glass sealing has the disadvantage that there may be large gradients in thermal expansion coefficients through the structure, even where the pin and the feedthrough plate are made of the same material (e.g., kovar). Temperature changes occurring during processing and service of the feedthrough create thermal stresses that can lead to failure and loss of hermeticity between the pin and the glass. Glass insulator structures typically have low yields for multiple-pin designs.
In another technique, green ceramic material is placed between the metallic pin and the bore in the ceramic feedthrough plate, and the assembly is heated to sinter the ceramic. This approach requires sintering at high temperature, which may be only difficultly compatible with the other fabrication and assembly steps. Moreover, experience has shown that the ceramic-sealed feedthroughs may lose hermeticity when thermally cycled in harsh environments during service.
There is a need for an improved technique for preparing electrical feedthroughs that produces a more robust structure. The present invention fulfills this need, and further provides related advantages.